U.S. patent number 9,541,510 [Application Number 13/685,835] was granted by the patent office on 2017-01-10 for system and methods for multi-beam inspection of cargo in relative motion.
This patent grant is currently assigned to American Science and Engineering, Inc.. The grantee listed for this patent is American Science and Engineering, Inc.. Invention is credited to Anatoli Arodzero, Martin Rommel.
United States Patent |
9,541,510 |
Arodzero , et al. |
January 10, 2017 |
System and methods for multi-beam inspection of cargo in relative
motion
Abstract
X-ray inspection of moving cargo based on acquiring multiple
image lines at one time or substantially at one time. An X-ray
source with multiple-beam electron beam targets creates multiple
parallel X-ray fan beams. X-ray inspection systems and methods
employ such multiple-beam sources for purposes of inspecting fast
moving cargo.
Inventors: |
Arodzero; Anatoli (Billerica,
MA), Rommel; Martin (Lexington, MA) |
Applicant: |
Name |
City |
State |
Country |
Type |
American Science and Engineering, Inc. |
Billerica |
MA |
US |
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Assignee: |
American Science and Engineering,
Inc. (Billerica, MA)
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Family
ID: |
48466868 |
Appl.
No.: |
13/685,835 |
Filed: |
November 27, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130136230 A1 |
May 30, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61564526 |
Nov 29, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01V
5/0016 (20130101); G01N 23/083 (20130101); G01N
23/046 (20130101); G01N 23/02 (20130101) |
Current International
Class: |
G01N
23/10 (20060101); G01N 23/02 (20060101); G01N
23/04 (20060101); G01V 5/00 (20060101); G01N
23/083 (20060101) |
Field of
Search: |
;378/57 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Arke et al., `A Solid-State Nanosecond Beam Kicker Modulator Based
on the DSRD Switch,` Aug. 2011, Conf. Proc. C11-03-28. cited by
examiner .
Jin Ho Park, Authorized officer Korean Intellectual Property
Office, International Search Report and Written Opinion of the
International Searching Authority--International Application No.
PCT/US2012/066612, dated Feb. 27, 2013 (12 pages). cited by
applicant.
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Primary Examiner: Kim; Robert
Assistant Examiner: Osenbaugh-Stewar; Eliza
Attorney, Agent or Firm: Sunstein Kann Murphy & Timbers
LLP
Parent Case Text
The present application claims priority from U.S. Provisional
Patent Application Ser. No. 61/564,526, filed Nov. 29, 2011, and
incorporated herein by reference.
Claims
What is claimed is:
1. A cargo inspection system employing a plurality of concurrent
fan beams of penetrating radiation, for inspecting cargo in motion
relative to the cargo inspection system, the cargo inspection
system comprising: a. a single source of a pulsed beam of
accelerated electrons; b. a plurality of Bremsstrahlung targets for
emitting penetrating radiation upon impingement by a spatial
entirety of the pulsed beam during each of a plurality of pulses of
the accelerated electrons; c. a plurality of collimators for
forming the emitted penetrating radiation into a plurality of
substantially parallel fan beams; d. a plurality of linear detector
arrays, each linear detector array for receiving penetrating
radiation transmitted through the cargo in a corresponding fan beam
of the plurality of fan beams; and e. a processor for deriving a
material characteristic of the cargo for each of a plurality of
lines of sight through the cargo.
2. A cargo inspection system in accordance with claim 1, wherein an
energy spectrum characterizing pulses of the penetrating radiation
varies between distinct pulses as a function of time.
3. A cargo inspection system in accordance with claim 1, further
comprising a speed sensor for providing a cargo speed to the
processor.
4. A method for deriving a specified characteristic of an inspected
object, the method comprising: a. generating, during a single pulse
of accelerated electrons, a plurality of concurrent substantially
parallel x-ray beams, by directing an entire electron beam onto a
succession of Bremsstrahlung targets; b. concurrently irradiating
the object with the plurality of concurrent substantially parallel
fan beams of penetrating radiation; c. detecting the penetrating
radiation after traversal of the inspected object with a plurality
of linear detector arrays, thereby generating a detector signal;
and d. processing the detector signal to derive the specified
characteristic of the inspected object.
5. A method in accordance with claim 4, further comprising
detecting each of the plurality of substantially parallel fan beams
with a distinct detector array of the plurality of detector arrays
in an interlaced manner.
6. A method in accordance with claim 4, wherein the penetrating
radiation is generated in a plurality of pulses, each pulse
characterized by an energy spectrum, and the plurality of pulses
characterized by a pulse rate.
7. A method in accordance with claim 6 wherein the energy spectrum
characterizing each of the plurality of pulses, varies from pulse
to pulse, as a function of time.
8. A method in accordance with claim 7, further comprising
interrogating a plurality of slices of the target with a plurality
of pulses, wherein temporally adjacent pulses are characterized by
distinct energy spectra.
9. A method in accordance with claim 6, further comprising
synchronizing relative motion of the inspected object with the
pulse rate.
Description
TECHNICAL FIELD
The present invention relates to methods and apparatus for cargo
inspection with penetrating radiation, and, more particularly, to
high speed, high throughput inspection systems employing pulsed
X-ray sources and providing enhanced material discrimination.
BACKGROUND ART
X-ray cargo inspection systems typically use an X-ray fan beam
generated by a pulsed high-energy X-ray source, such as a linear
accelerator (linac) or a betatron. The highest available pulse
rates from these sources limit the line frequency of the imaging
system and thus the maximum scan speed for a given line resolution.
Linear accelerators are available with pulse rates up to 1000
pulses per second (pps). At that rate an object with a speed of 60
km/h moves 16.7 mm per pulse. In order to achieve a typical 4 mm
vertical line pair resolution, four image lines must to be acquired
simultaneously. Employing multiple sources with multiple detector
arrays, however, is a costly proposition.
FIG. 1 depicts a cargo inspection system employing an x-ray
transmission technique. A fan-shaped beam 12 of penetrating
radiation, emitted by a source 14, is detected by elements of a
detector array 16 distal to a target object, here truck 10, in
order to produce images of the target object. Particular contents
of the object may be discriminated and characterized on the basis
of the transmission of penetrating radiation through the object and
its detection by detector array 16 and its individual detector
modules 18. (As used herein, the term "detector module" refers to a
detector element in conjunction with its associated preprocessing
electronics.) Signals from each of the detector modules, suitably
pre-processed, provide inputs to processor 19, where material
characteristics are computed.
Information (such as mass absorption coefficient, effective atomic
number Z.sub.eff, electron density, etc.) regarding the material
composition of the contents of objects may be obtained on the basis
of the interaction of X-rays with the material, and, more
particularly, by illuminating the material with X-ray beams having
energy spectra with more than one distinct energy endpoint (peak
energy), or by employing energy discriminating detectors. Dual
energy methods of material discrimination are widely used in X-ray
inspection systems for security control of hand luggage in customs
and other security checkpoints. Dual energy inspection is discussed
in the following references, for example, which are incorporated
herein by reference: U.S. Pat. No. 5,524,133, Neale et al.,
"Material Identification using X-Rays" (1996) (hereinafter, "Neale
'133") U.S. Pat. No. 7,257,188, Bjorkholm, "Dual Energy Scanning of
Contents of an Object" (2005) U.S. Pat. No. 6,069,936, Bjorkholm,
"Material Discrimination using Single-Energy X-Ray Imaging System"
(2000)
A multi-view x-ray inspection system is disclosed in US Published
Patent Application US 2011/0206179 ("Bendahan"), incorporated
herein by reference, which suggests rapidly steering a single
electron beam to a sequence of x-ray radiation-producing targets,
and shows an embodiment in which a beam appears to be detected by
multiple parallel detector arrays, although this embodiment is not
described in detail.
SUMMARY OF EMBODIMENTS OF THE INVENTION
In accordance with embodiments of the present invention, a cargo
inspection system is provided that employs a plurality of fan beams
of penetrating radiation, for inspecting cargo in motion relative
to the cargo inspection system. The cargo inspection system has a
source of a beam of accelerated electrons, at least one
Bremsstrahlung target for emitting penetrating radiation upon
impingement by the accelerated electrons, and a plurality of
collimators for forming the emitted penetrating radiation into a
plurality of substantially parallel fan beams. Additionally, the
cargo inspect system has a plurality of linear detector arrays,
where each linear detector array receives penetrating radiation
transmitted through the cargo in a corresponding fan beam, and a
processor for deriving a material characteristic of the cargo for
each of a plurality of lines of sight through the cargo.
In other embodiments of the present invention, the plurality of fan
beams may be parallel beams, and may be emitted in planes
substantially transverse to the beam of accelerated electrons.
Alternatively, the plurality of fan beams may be emitted from the
at least one Bremsstrahlung target in a substantially forward
direction with respect to the beam of accelerated electrons. This
may be accomplished by the beam of accelerated electrons impinging
upon each of a plurality of Bremsstrahlung targets at slightly
different angles, or by fanning out the beam of accelerated
electrons and refocusing upon each of a plurality of Bremsstrahlung
targets.
In accordance with further embodiments of the present invention,
the source may be configured such that the beam of accelerated
electrons impinges upon a plurality of Bremsstrahlung targets
either simultaneously or in sequence.
In yet further embodiments, an energy spectrum characterizing the
penetrating radiation may vary as a function of time.
The system may also have a speed sensor for providing a cargo speed
to the processor.
In accordance with another aspect of the present invention, a
method is provided for deriving a specified characteristic of an
inspected object, the method has processes including: a.
irradiating the object with a plurality of substantially parallel
fan beams of penetrating radiation; b. detecting the penetrating
radiation after traversal of the inspected object with a plurality
of linear detector arrays, thereby generating a detector signal;
and c. processing the detector signal to derive the specified
characteristic of the inspected object.
The steps of associating the plurality of detector arrays with the
plurality of fan beams may be performed in an interlaced pair-wise
manner.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of the invention will be more readily
understood by reference to the following detailed description,
taken with reference to the accompanying drawings, in which:
FIG. 1 is a perspective view of a prior art x-ray transmission
cargo inspection system.
FIG. 2 is a horizontal plane cross-sectional schematic of cargo
under inspection using a single beam and a two-dimensional detector
array, viewed in the rest frame of the cargo.
FIG. 3 is a schematic depiction of inspection by means of quadruple
beams per source.
FIG. 4 shows extraction of multiple parallel fan beams transverse
to an incident electron beam, in accordance with an embodiment of
the present invention.
FIG. 5 is a cross-sectional schematic view of an apparatus for
generating multiple parallel fan beams in planes to which the
incident electron beam is parallel, in accordance with an
embodiment of the present invention;
FIG. 6 is a cross-sectional schematic view of another apparatus for
generating multiple parallel fan beams in planes to which the
incident electron beam is parallel, in accordance with an
embodiment of the present invention;
FIG. 7 is a cross-sectional schematic view of another apparatus for
generating multiple parallel fan beams from multiple targets to the
incident electron beam is switched in succession during the course
of a single pulse, in accordance with an embodiment of the present
invention;
FIG. 8 depicts an interlacing scheme applicable to three parallel
fan beams, where three detector arrays are spaced one beamwidth
apart, in accordance with an embodiment of the present
invention;
FIG. 9 depicts an interlacing scheme applicable to three parallel
fan beams, where three detector arrays are spaced four beamwidths
apart, in accordance with an embodiment of the present
invention;
FIG. 10 depicts an interlacing scheme applicable to four parallel
fan beams operating at two energies, where four detector arrays are
spaced two beamwidths apart, in accordance with an embodiment of
the present invention; and
FIG. 11 shows a perspective view of a cargo inspection system in
accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION
Definitions.
As used herein and in any appended claims, the term "beam" refers
to a flux of particles (including photons or other massless
particles) having a predominant direction referred to as the
direction of the beam. Any plane containing the direction of the
beam may be referred to as a plane of the beam.
The term "multiple targets" encompasses the case of a single target
which is impinged upon at distinct, non-contiguous regions, thereby
generating multiple beams.
The term "image" shall refer to any multidimensional
representation, whether in tangible or otherwise perceptible form,
or otherwise, whereby a value of some characteristic (such as
fractional transmitted intensity through a column of an inspected
object traversed by an incident beam, in the case of x-ray
transmission imaging) is associated with each of a plurality of
locations (or, vectors in a Euclidean space, typically )
corresponding to dimensional coordinates of an object in physical
space, though not necessarily mapped one-to-one thereonto. An image
may comprise an array of numbers in a computer memory or
holographic medium. Similarly, "imaging" refers to the rendering of
a stated physical characteristic in terms of one or more
images.
The term "image line" refers to a one-dimensional image obtained on
the basis of a linear detector array upon illumination by a fan
beam.
The term "concurrent X-ray beams," as used herein and in any
appended claims, refers to multiple beams that exist within a time
scale defined by the duration of a source pulse.
Advantages associated with feeding multiple detector arrays with a
single source, as opposed to multiple sources, particularly in the
field of x-ray inspection, may be achieved in accordance with
various embodiments of the present invention.
One approach to x-ray cargo inspection that employs a single source
and multiple detector arrays creates a single wide fan beam which
covers a detector array N-pixels wide. Such a system, where, by way
of example, N=4, is depicted in FIG. 2. The embodiment of FIG. 2 is
not preferred, however, for reasons that will be discussed. The
schematic cross-section shown in FIG. 2 is depicted in the frame of
reference of cargo 20, which may be a container, or a truck, etc.
X-ray beam 22 emerges from a collimator which constitutes part of
source 14 of x-ray radiation. X-ray beam 22 traverses cargo 20,
thereby interrogating an intervening segment 21 of the contents of
cargo 20. Assuming that source 14 is pulsed, N=4 line images are
obtained during each pulse from the incidence of beam 22 on
detectors 23, which represent a section, in the plane of the page,
of linear detector arrays extending up and down in a plane
transverse to the plane of the page.
During the temporal interval between successive pulses, cargo 20 is
displaced by distance 24, such that during the succeeding pulse,
segment 25 of the cargo 20 is interrogated. It should be noted
that, in accordance with this scheme, due to the quiescent interval
between pulses, a region 26 fails to be interrogated at all.
Several deficiencies may detract from the approach depicted in FIG.
2, including the five deficiencies enumerated here: 1. The four
image lines are projections with slightly different angles which
leads to distortions unless the source is very far away; 2. There
is very little space for the required photo-detectors in a 2D
array; 3. At the required high X-ray energies there is significant
cross talk between neighboring scintillator crystals, as discussed,
for example, by Descalle, et al., in "Detector design for
high-resolution MeV photon imaging of cargo containers using
spectral information," Nucl. Instruments and Methods in Phys. Res.
A, vol. 624, pp. 635-40 (2010), incorporated herein by reference;
4. The wide beam 22 will result in increased scatter contributions
relative to a narrow fan beam. 5. Some volume 26 of cargo will be
not interrogated.
The deficiencies numbered 2 through 4 above can be avoided by
creating multiple narrow fan beams from a single source. In that
embodiment, each fan beam remains paired with a separate detector
array. However, the approach of multiple narrow beams aggravates
item 1 above. Now that the angle between the beam planes is even
larger and discontinuous, overlapping projections are created. FIG.
3 illustrates the foregoing problem for a source with quadruple fan
beams 31, 32, 33, and 34 per source. (Note that the aspect ratio is
on the order of 100:1).
Creating Parallel Fan Beams
A solution preferable to that of FIG. 3 for very high speed
imaging, and a solution which addresses all of the five previously
enumerated issues, is that of employing a plurality of
substantially parallel fan beams created by a single accelerator.
In accordance with embodiments of the present invention, an
accelerator is provided having multiple targets for one electron
beam. Each target creates a source of Bremsstrahlung so that
parallel fan beams can be created with a multi-slit parallel
collimator.
One such embodiment is now described with reference to in FIG. 4.
Electron beam 41 is incident upon successive targets 43, and
Bremstrahlung radiation 44 emitted substantially transverse to
electron beam 41 is collimated by parallel collimators 45 into
substantially parallel fan beams. It is to be noted that, for a 9
MeV electron beam 41 impinging on a thin target 43, the X-ray flux
(per unit solid angle, dI/d.OMEGA.(.theta.)) 44 near 90.degree. is
only approximately 6% of the forward (0.degree.) flux. For a 6 MeV
electron beam that value is approximately 7%. Since only a small
fraction of the total generated flux is utilized and is shared
among multiple X-ray beams, a relatively large source current is
required for the illustrated approach.
In an alternative embodiment of the present invention, described
with reference to FIGS. 5 and 6, electron beam 50 is directed in
line with (i.e., either within, or parallel to) the X-ray beam
planes (the planes containing fan beams 51). Electron beam 50 is
fanned out in the plane of the page by beam expander 52 so as to
cover multiple targets 53. Any of the well-known techniques of
control and expansion of charged particle beams by electromagnetic
fields, or those yet to be developed, are within the scope of the
present invention. If, as in the case shown in FIG. 5, no further
beam optics are applied, only a fraction of the electron flux is
utilized for generating the X-ray beams. However, since electron
beam and X-ray beam differ only by a small angle, the strong
forward lobe of the angular Bremsstrahlung distribution is used
efficiently. Fan beams 51 emerging from respective targets are
separately collimated by parallel collimator 56. Thus, the forward
directed Bremsstrahlung of FIGS. 5 and 6 is more efficient than the
transverse emission embodiment of FIG. 4.
A significantly higher utilization of the electron flux can be
achieved by separating the fanned out electron beam into multiple
finger beams 61, each focused by beam focuser 63 to impinge upon
its designated target 62 as shown in FIG. 6.
Another embodiment, for distributing the original electron beam 50
over multiple targets 53 employs switching, that is, redirecting
the entire electron beam 50 onto one target 62 at a time.
U.S. Pat. No. 6,009,146, to Adler et al., describes moving an
electron beam magnetically between multiple targets with stationary
collimators to sequentially create multiple pencil beams of X-rays.
The term "concurrent X-ray beams," as used herein and in any
appended claims, refers to beams that exist within a time scale
defined by the duration of a source pulse. To create multiple
"concurrent" X-ray beams, within the foregoing meaning, a charged
particle beam is switched by fast beam switcher 70, at a rate
significantly higher than the pulse rate, as shown in FIG. 7. The
target 62 for the electron beam 71, is thus also switched at a rate
significantly higher than the pulse rate such that the electron
beam is redirected many times during the duration of a single X-ray
pulse, which typically lasts several microseconds. Beam-steering
switchers (or "kickers") that operate on nanosecond time scales
have been developed, and are described, for example, in the
following publications, all incorporated herein by reference:
Lambertson, "Dynamic Devices--Pickups and Kickers," in Physics of
Accelerators, (eds. M. Month and M. Dienes), AIP Conference
Proceedings 153, p. 1414 (1987); Goldberg, et al., "Dynamic
Devices: A Primer on Pickups and Kickers", in The Physics of
particle Accelerators, eds. M. Month and M. Dienes, AIP Conference
Proceedings 249, v. 1, p. 537 (1990); Krasnykh, "Development of a
Fast High-Power Pulser and ILC DR Injection/Extraction Kicker",
SLAC-WP-077, presented at ILC Damping Ring R&D Workshop, Sep.
26, 2007-Sep. 28, 2007, Cornell University, Ithaca, N.Y. (2007);
and Akre, et al., "A Solid-State Nanosecond Beam Kicker Modulator,
Based on the DSRD Switch". SLAC-PUB-14418. Particle Accelerator
Conference, (PAC'2011), New York, 2011; Poole et al., "Analysis and
Modeling of a Stripline Beam Kicker and Septum," International
Linear Accelerator Conference, Chicago, Ill., Aug. 23-28, 1998.
The fast beam switcher (kicker) 70 directs the entire electron beam
onto the individual targets at a very high rate. The electron beam
in a linear accelerator is not a steady stream of electrons but
consists of a series of so-called micro bunches. To ensure the best
utilization of the electron beam, the fast beam switching should be
timed so that the switching of targets occurs between micro
bunches. This is facilitated by linking the fast beam switcher to
the same GHz frequency which drives the accelerator.
Multi-Beam Interlacing Schemes
One objective of embodiments of the present invention is that of
acquiring a transmission X-ray image with complete coverage by
equidistant scan lines. Multiple parallel beam planes paired with
detector lines will produce multiple scan lines at a time. In order
for these scan lines to produce a complete image, the detector
lines need to be arranged with specified spacings and the pulse
rate of the X-ray source needs to correspond to the speed of the
object.
A multi-beam interlacing scheme in accordance with the foregoing
considerations may be implemented with any number NB of fan beams,
and the spacing of the detector lines depends on this number. For
the case of three fan beams, for instance, the detector lines could
be spaced one detector width DW apart or four DW apart. This is
illustrated in FIG. 8 and FIG. 9, respectively. In these figures,
time progresses from top to bottom, while the horizontal dimension
represents space, with the motion of the inspected cargo designated
by the numeral 80. Shading indicates that the respective cargo
region has already been scanned.
In general, the minimum spacing between detector lines in units of
DW is equal to NB-2, i.e., two fewer than the number of fan beams.
The detector line spacing can be increased by multiples of the
distance NB*DW.
The pulse rate in a multi-beam system is tuned to the speed of the
imaged object (scan speed) in order to space the scan lines evenly
over the imaged object.
.times..times..times..times. ##EQU00001##
Equivalently, the maximum scan speed of the system is the product
of the maximum pulse rate, the number of fan beams NB and the width
of the detector DW: Scan Speed=NB*DW*Pulse Rate
So for instance a quad beam system with 1 cm wide detectors working
at 400 pps allows scanning at 57.6 km/h. A quad beam system with 4
mm wide detectors needs to operate at 1000 pps for the same
scanning speed.
Material Discrimination with a Multi-Beam System
Various approaches known in the art to acquire data which allow
material discrimination are all compatible with the multi-beam
concept.
The most commonly applied dual energy method interlaces low and
high end-point energy pulses in time. The image is composed by
combining adjacent low and high energy pulses which effectively
reduces the pulse rate by a factor two. A second disadvantage is
that the low and the high energy scan lines being combined do not
originate from exactly the same but neighboring regions in the scan
object. Both these disadvantages can be addressed with the
multi-beam approach. If the source provides alternating low and
high energy pulses and an even number of fan beams is used, simply
scanning at a speed of Scan Speed=1/2NB*DW*Pulse Rate ensures that
each cargo region will be scanned once with a low, and once with a
high, end-point energy pulse. An example with four beams is shown
in FIG. 10 where the two energies are represented by different
hashing.
The other well established dual energy method uses detector
elements which provide two differently filtered signals and thus
enables material discrimination. This method is directly applicable
with multiple beams.
The Scintillation-Cherenkov detector and method for high energy
X-ray imaging disclosed in US Published Patent Application
2011/0163236 (Arodzero), incorporated herein by reference, and
intrapulse multi-energy and adaptive multi-energy methods of cargo
inspection disclosed in US Published Patent Application
2012/0093289 (Arodzero et al.), both incorporated herein by
reference, are also directly applicable with the presently
described multi-beam cargo inspection methods.
Further new methods are enabled with multiple beams, as the
distinct beams may be filtered differently at the source.
Alternatively, detector designs with differing energy sensitivities
can be employed for different beams in the multi-beam system.
FIG. 11 shows a perspective view of a multi-beam cargo inspection
system in accordance with an embodiment of the present invention.
The electron beam of a linac-based X-ray generator 110 is switched
by electron beam switcher 70 to multiple targets 62 (shown in FIG.
7) generating multiple beams 116 collimated by multi-slot
collimator 114. Each beam corresponds to a linear array 118 of
detectors on the opposing side of inspected cargo 20. A processor
115 receives detector signals from each detector module comprising
the various linear detector arrays 118 and computes a relative
transmission associated with each line of sight through the cargo
20. A speed sensor 120 may provide a signal to processor 115 for
synchronizing cargo speed and source frequency to achieve specified
resolution.
Where examples presented herein involve specific combinations of
method acts or system elements, it should be understood that those
acts and those elements may be combined in other ways to accomplish
the same objective of providing a multiple x-ray fan beams from a
single source. Additionally, single device features may fulfill the
requirements of separately recited elements of a claim. The
embodiments of the invention described herein are intended to be
merely exemplary; variations and modifications will be apparent to
those skilled in the art. All such variations and modifications are
intended to be within the scope of the present invention as defined
in any appended claims.
* * * * *